1. Field of the Invention
The present invention is directed to an apparatus for charging living tissue with electrical pulses of the type having a charging capacitor connectable across a charging circuit and connectable via a controllable switch stage to at least two electrodes arranged in the region of the tissue for discharge therethrough and having a control arrangement that opens and closes the switch with a prescribed switching frequency.
2. Description of the Prior Art
An apparatus of the above type is disclosed by German OS 37 15 822 and serves for the defibrillation or cardioversion of a heart. In this known apparatus, a charging capacitor that was previously charged to a prescribed charging voltage by a charging circuit is connected via a controllable switch stage to two electrodes arranged in the region of the heart. The controllable switch stage is switched open and closed by a control means with a prescribed, fixed frequency in the range from 10 kHz through 1 MHz, as a result of which the defibrillation current through the heart is divided into a multitude of individual pulses following one another with a high repetition rate and whose pulse height exponentially decays. By charging the heart with the high-frequency pulses, the frequency dependency of the impedance of the heart tissue is to be utilized for distributing the defibrillation energy over the entire heart, so that a low energy is required for effective defibrillation.
U.S. Pat. No. 4,834,100 discloses another apparatus for defibrillation of a heart, wherein the current pulse supplied to the heart has the course of a highly attenuated sine oscillation (referred to as a Lown wave) that is viewed as being especially effective for achieving a defibrillation of the heart. This curve of the current is realized by arranging an inductance in the discharge circuit of a charging capacitor connected to electrodes in the region of the heart via a controllable switch means. Since the charging capacitor has a high value of capacitance, the inductance must have a correspondingly high value of inductance in order to obtain the desired current curve. This, however, involves comparatively large physical dimensions of the inductance, which is disadvantageous, particularly given implantable defibrillators. Moreover, the curve of the current in the known defibrillator is permanently prescribed by the selected values of capacitance and inductance, and thus the influence of the impedance of the heart tissue and the arrangement of the electrodes (which may be different from patient to patient) on the curve of the current cannot be taken into consideration.
Heretofore, a series of different pulse shapes such as, for example, the two aforementioned pulse shapes as well as square-wave pulses or exponentially decaying current pulses have been investigated with respect to their efficiency for defibrillalting or cardioverting a heart. The shape of the respective current pulses investigated was thereby predominantly predetermined by the technological possibilities for generating the pulses.